1. Field of the Invention
The present invention relates to frequency bands for communication systems, and more particularly to a method for transmitting and receiving data by use of multiple bands.
2. Description of the Related Art
In general, communication systems use a certain frequency band to transmit data. The data used for the communication systems is divided into circuit data and packet data. The circuit data is data such as voice signals that has to be transmitted and received in real time, whereas the packet data has a data amount of over a certain size such as packet information and does not have to be transmitted at real time. The frequency band used for transmission of the circuit data is narrow in general. However, a wider frequency band is required to transmit the packet data.
As stated above, the frequency band to be used also increases as an amount of data to be transmitted increases. Hereinafter, the wider frequency band is referred to as the ultra wide band (UWB). The UWB is divided into plural frequency sub-bands each having a certain size. The communication system transmits data using the plural frequency sub-bands in a specific period of time, so a lot of data can be transmitted for the specific period of time. The communication system selects one of the plural frequency sub-bands for the specific period of time, and transmits data using the selected frequency sub-band, to thereby enhance data security. That is, the communication system can enhance data security by sequentially using the plural frequency sub-bands.
FIG. 1 is a view for showing a structure of the UWB currently proposed. As shown in FIG. 1, the frequency band of the currently proposed UWB lies in a frequency range from 3432 MHz to 10032 MHz. The frequency band of the UWB largely consists of four groups denoted as group A to group D, with group D having four frequency sub-groups.
The reference frequencies of three frequency sub-bands of group A are 3432 MHz, 3960 MHz, and 4488 GHz, and the reference frequencies of two frequency sub-bands of group B are 5016 MHz and 5808 MHz. The four reference frequencies of group C are 6336 MHz, 6864 MHz, 7392 MHz, and 7920 MHz, and the four reference frequencies of group D are 8448 MHz, 8976 MHz, 9504 MHz, and 10032 MHz. The frequency sub-bands included in group B are overlapped with the frequency bands occupied by current wireless LANs, and it is impossible to use the frequency sub-bands included in the group D in view of the level of current technologies. Accordingly, methods are being currently discussed to use the frequency sub-bands of group A and the frequency sub-bands of group C.
Seven reference frequencies have to be produced to use the three frequency sub-bands of group A and the four frequency sub-bands of the group C. That is, a structure is needed to produce the seven reference frequencies in order to use the three frequency sub-bands of group A and the four frequency sub-bands of group C.
The communication system generates a radio frequency (RF) by use of an intermediate frequency (IF) and the reference frequencies of a local oscillator (LO) in order to transmit data, and transmits data by use of the generated radio frequency. The seven reference frequencies are generated in the local oscillator. Hereinafter, the local oscillator is described to generate the seven reference frequencies.
FIG. 2 is a view for showing local oscillators for generating the seven reference frequencies. As shown in FIG. 2, seven local oscillators are needed to generate the seven reference frequencies. That is, one local oscillator generates only one reference frequency. Phase locked loops (PLLs) work to stabilize the reference frequencies generated from the local oscillators. Thus, it is structured to have one local oscillator corresponding to one PLL. Hereinafter, a process for generating the seven reference frequencies is described by use of FIG. 2.
The local oscillator 200 generates a reference frequency 3432 MHz, and the local oscillator 201 generates a reference frequency 3960 MHz. The local oscillator 206 generates a reference frequency 7920 MHz. The PLL 210 stabilizes the reference frequency generated from the local oscillator 200, and then transmits the reference frequency to the selection unit 220. The PLL 211 stabilizes a reference frequency generated from the local oscillator 201, and then transmits the reference frequency to the selection unit 220. The PLL 216 stabilizes a reference frequency generated from the local oscillator 206, and then transmits the reference frequency to the selection unit 220. The selection unit 220 selects one of the stabilized reference frequencies transmitted according to a control signal, and outputs the selected reference frequency. The reference frequency outputted from the selection unit 220 is combined with the IF signal, and then converted to the RF signal.
Seven local oscillators require seven PLLs in order to generate the seven reference signals as shown in FIG. 2. However, the local oscillators and PLLs have a disadvantage of having a high power consumption. Furthermore, the local oscillators and PLLs have a disadvantage of increasing the size of a communication system.